Compute an approximated sample size for linear mixed models (two-level-designs), based on power-calculation for standard design and adjusted for design effect for 2-level-designs.
samplesize_mixed( eff.size, df.n = NULL, power = 0.8, sig.level = 0.05, k, n, icc = 0.05 ) smpsize_lmm( eff.size, df.n = NULL, power = 0.8, sig.level = 0.05, k, n, icc = 0.05 )
Optional argument for the degrees of freedom for numerator. See 'Details'.
Power of test (1 minus Type II error probability).
Significance level (Type I error probability).
Number of cluster groups (level-2-unit) in multilevel-design.
Optional, number of observations per cluster groups (level-2-unit) in multilevel-design.
Expected intraclass correlation coefficient for multilevel-model.
A list with two values: The number of subjects per cluster, and the total sample size for the linear mixed model.
The sample size calculation is based on a power-calculation for the
standard design. If
df.n is not specified, a power-calculation
for an unpaired two-sample t-test will be computed (using
pwr.t.test of the pwr-package).
df.n is given, a power-calculation for general linear models
will be computed (using
pwr.f2.test of the
pwr-package). The sample size of the standard design
is then adjusted for the design effect of two-level-designs (see
design_effect). Thus, the sample size calculation is appropriate
in particular for two-level-designs (see Snijders 2005). Models that
additionally include repeated measures (three-level-designs) may work
as well, however, the computed sample size may be less accurate.
Cohen J. 1988. Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale,NJ: Lawrence Erlbaum.
Hsieh FY, Lavori PW, Cohen HJ, Feussner JR. 2003. An Overview of Variance Inflation Factors for Sample-Size Calculation. Evaluation and the Health Professions 26: 239-257. doi: 10.1177/0163278703255230
Snijders TAB. 2005. Power and Sample Size in Multilevel Linear Models. In: Everitt BS, Howell DC (Hrsg.). Encyclopedia of Statistics in Behavioral Science. Chichester, UK: John Wiley and Sons, Ltd. doi: 10.1002/0470013192.bsa492
# Sample size for multilevel model with 30 cluster groups and a small to # medium effect size (Cohen's d) of 0.3. 27 subjects per cluster and # hence a total sample size of about 802 observations is needed. samplesize_mixed(eff.size = .3, k = 30) #> $`Subjects per Cluster` #>  27 #> #> $`Total Sample Size` #>  802 #> # Sample size for multilevel model with 20 cluster groups and a medium # to large effect size for linear models of 0.2. Five subjects per cluster and # hence a total sample size of about 107 observations is needed. samplesize_mixed(eff.size = .2, df.n = 5, k = 20, power = .9) #> $`Subjects per Cluster` #>  5 #> #> $`Total Sample Size` #>  107 #>